The present invention relates to a variable-output pump is general, and more particularly to a control arrangement for controlling the output of such a variable-output pump.
Variable-output pumps are already known in the art and, generally speaking, they include an output-adjusting element, be it an inclined plate of an axial piston machine, an eccentric ring of a radial piston machine or similar adjusting elements of other types of pumps. The functional arrangement of the present invention can be used in connection with any of such conventional variable-output pumps, regardless of the particular construction of the pump and the shape, mounting and displacement of the adjusting element. It is sufficient for understanding the present invention to mention that the adjusting element of such a conventional variable-output pump in which the control arrangement of the present invention is to be used is to be displaced in mutually opposite directions in order to increase or decrease the output of the variable-output pump.
There are also already known various arrangements for varying the position of the adjusting element of the variable-output pump in dependence on the operating parameters of the pump. So, for instance, it is well known to use a differential piston arrangement for displacing the adjusting elements between the low-output and high-output position thereof, depending on the load consumed by a user. Such differential piston arrangement usually includes two pistons, one of a larger and another of a smaller active area, which may be separate or combined into a single piston. Such pistons act on the adjusting element of the variable-output pump and displace the same between the low-output and high-output positions. Pressurized control fluid is permanently supplied to the piston of a smaller active area, and only selectively to the piston of the larger active area. A spring may act against the force of the control fluid acting on the piston of a smaller active area and displace the adjusting element toward the low-output position thereof until the pressure of the control fluid exceeds a predetermined threshold value.
There have also been already proposed various arrangements for controlling the selective admission of the control fluid to the piston having the larger active area, such control arrangements usually including a slide valve mounted in a valve housing and assuming different positions depending on the pressure of the control fluid. Under most circumstances, such control slide is a two-position valve which either admits the pressurized control fluid to the piston having the larger active area, or discharges the control fluid from this piston into a relief conduit. However, there have been also proposed some more elaborate slide valve arrangements of different constructions.
The control arrangements for variable-output pumps must satisfy several requirements. First of all, the control operation must be conducted rather rapidly. In other words, the control arrangement must quickly respond to the changes of the operating conditions, such as the load of the variable-output pump, the discharge pressure in the conduit between the variable-output pump and the user of the fluid, or other operating parameters. On the other hand, it is an additional requirement that the amount of hysteresis be rather low, irrespective of the rather high amount of frictional losses in the control arrangement.
In order to satisfy such requirements, it has been already proposed to use a proportional control arrangement. However, experience with this type of an arrangement has shown that a high degree of amplification must be used therein in order to be able to satisfy the above conditions. Furthermore, as a result of the high friction of the elements which take part in the adjusting of the output of the variable-output pump, the control arrangement has a very low stability. When it is attempted to reduce this instability by increasing the damping, the control arrangement becomes too slow.
The above-mentioned conditions and requirements could theoretically be satisfied by using a proportional-differential control arrangement or a proportional-integral control arrangement. However, such control arangements are very complex, expensive and prone to malfunction.